The present invention relates to a handoff technology of wireless communication and a method of mobility management. More particularly, the present invention mainly relates to a handoff method of wireless LAN.
Handoff management is also an initially tested procedure for ensuring a seamless and lossless handoff during a procedure for moving a mobile terminal from a base station to another base station. The handoff technology is the critical technology therein, and directly affects the moving capability of the mobile terminal.
Handoff includes several procedures: measurements, decision, and performing of the handoff. The handoff can be distinguished according to several criteria:
a. Location of Handoff Function
Here, it is distinguished between network-initially-tested handoff (whether the terminal assisted or the whole network for making the control) and terminal-initialized handoff (or decision by both together).
b. Involved Network Element
Intra-signal-cell between signal cells and between networks.
c. Number of Effective Connections
Hard handoff, only one effective connection existing, or soft handoff being allowed to simultaneously maintain several connections.
d. Types of Data Transmission: Circuit Switched or packet switched data
The performance requirements of handoff are as follows:                Small latency: in order to satisfy the moving speed of the mobile terminal and property of the transmitted data, the needed handoff time becomes as small as possible;        Scalability: the handoff procedure should support the handoffs within the same intra-signal-cell, between different base stations and between the same or different networks;        Minimal drop-off packet and fast recovery;        Maintaining quality of service or re-negotiating after completion of handoff; and        Minimal additional signaling.        
The performance of the handoff method is the primary concerning problem when the mobile equipment is moving in the wireless communication networks.
The conventional wireless handoff method refers to Chi-Chun Lo and Ming-Hua Lin, “QoS Provisioning in Handoff Algorithms for Wireless LAN”, Broadband Communications, 1998, Accessing, Transmission, Networking. Proceedings, International Zurich Seminar, 17-19 Feb. 1998, Page(s): 9-16., which is mainly based on the strength of measured signals and performing some scaling as follows:
(1) Maximum power handoff: In this method, mobile terminal is selectively connected to a base station, which has the maximum received power.
(2) Relative average received power: When the averaged value of signal power received from a new base station within a time period is greater than the current base station, handoff is performed.
(3) According to the relative received power of Hysteresis sideband: When the power of the received signal is greater than that of the signal of the current base station by a predetermined value of Hysteresis sideband, then it is handoffed to the new base station.
(4) According to the absolutely received power of a threshold value: In this method, if the signal of the new base station is greater than the threshold value and the signal of the current base station is less than the threshold value, the handoff is performed.
(5) According to the relatively received power of the threshold value: In this method, when the signal of the current base station is less than the threshold value and if the signal of the new base station is greater than the threshold value or greater than the signal of the current base station, the handoff is performed.
(6) Maximum Power Handoff with Timer: At the nth sampling point of the received power, if the new base station is greater than the current base station, then a time value (t) is set and it is handoffed if the situation remains the same after elapsing this time period.
In addition, a handoff method, for example, Toni Janevski and boris Spasenovski, “A Novel QoS Scheme for Handoffs in Wireless IP Networks”, Wireless Communications and Networking Conference, WCNC. 2000 IEEE, 2000, pp. 496-501 vol. 2, supports the mobility in wireless mobile-phone communication. However, since the wireless LAN and the wireless mobile-phone communication are intrinsically different, the handoff method and the QoS parameter in wireless mobile-phone communication cannot be directly applied to the wireless LAN.
A set of service is specified in IEEE802.11 wireless LAN Standard (see Wireless LAN Medium Access Control (MAC) and Physical layer (PHY) specifications: Higher-Speed Physical Layer Extension in the 2.4 GHz Band, IEEE draft standard 802.11b, 1999), wherein reassociation service is capable of a supporting mobility function.
The association service has been sufficient for the handoff between IEEE802.11 base stations with no-transition. For the handoff of BSS-transition, it then needs a new function of reassociation service in the wireless LAN. Reassociation is a distribution system service (DSS).
When the mobile terminal moves from one access point to another access point, it immediately triggers the reassociation service. This lets the distribution system DS inform a map between the current access point and the terminals, moving from the basic service set to another one of basic service set in the same one of scaled service set. The reassociation causes a termination of the current association relation between the access point and the base station, and then a new association is built up. Usually, the mobile terminal activates the automatic reassociation service.
Thus, according to the foregoing definition, a summary is obtained that 802.11 is a hard handoff procedure triggered by the mobile terminal. A signal beacon is used for synchronization and measurements between the base stations.
Anyway, since the IEEE802.11 Wireless LAN Standard does not specify a handoff method, as it is now, each provider is always based on strength of the received signal to ensure the handoff. This then causes the fact that all of these handoff methods are not considering the requirement of QoS, and it surely causes some problems, such as the unnecessary handoff. In some handoff methods in the mobile-phone communication, the association time and system load have ever been considered. However, up to now, the handoff algorithm in the wireless LAN is not considered with respect to these two parameters. The handoff algorithm in the wireless LAN is still relatively simple.
Due to the competing access method of CSMA/CA in wireless LAN being rather sensitive to the system load, the heavier system load has degraded the system performance. At the same time, phenomena of multiple paths in room and so on always cause the mobile terminal being back-and-forth handoff between two access points, and the unnecessary handoff. The inventors of this application propose a handoff method of wireless LAN. In the method, an association time, a signal strength, and system load are simultaneously considered so as to prevent the unnecessary handoff and improve QoS performance in combination.